The present invention relates to infrared detection methods, and relates more particularly to improvements in such detection methods whereby the method and circuitry to perform the corrective signal processing to compensate for known or intrinsic optical signal intensity errors to the image plane of systems using electronics electro-optics sensors is provided to detect a thermal signature which would otherwise be less detectable.
In modern electro-optical sensor systems, solid-state electronic devices are used to perform the function of sensing incident radiation at the image plane of the system, integrating this signal, and multiplexing it for processing by systems electronics. Examples of the solid-state electronic devices are the charge coupled device (CCD) used in video cameras and infrared focal plane arrays that are used in many civilian and military systems. For these arrays two principal component materials are used to realize their fabrication. These are the incident radiation sensing detector material and the readout integrated circuit. The detector materials is chosen and optimized for sensing specific incident radiation wavelengths, and the readout is selected for its properties in realizing the desired signal processing to multiplexing functions.
Novel signal processing circuitry is described that allows correction of optical system non-uniformities to be performed on focal plane array. In effort to reduce the size and weight of modern infrared focal plane array systems, slower f-number optical systems with shorter cold shields are often used. In these systems, the signal and background intensity levels at the image focal plane decrease with increasing distance from the optical center axis. This results in a situation where some of the signal dynamic range is lost due to the off-axis optical effects. The incorporation of on-focal plane array signal processing electronics that correct for these off axis optical effects allows the full dynamic range of the sensor to be achieved, thus allowing a lower-cost and higher performance system to be realized.
In previous scanning and staring focal plane arrays the response to incident radiation (signal chain transimpedance) have been designed such that it is the same for each detector channel. In these systems and in an ideal sense, an incident signal that is non-uniform will produce an output signal from the sensor that has the same non-uniform characteristic. It is possible, however, to calculate based on the design of the optical system, the optical signal intensity errors that will be present. In the case of many infrared systems the signal intensity that reaches the focal plane array decreases on the focal plane with distance from the optical center axis. This effect typically results in a decrease in signal level of 20 percent or more from a signal at the optical center axis. This signal loss therefore accounts for more than 20 percent of the available dynamic range for the sensor output. The off-axis signal intensity error increases with slow optics, shorter cold shields, and larger focal plane arrays. The signal loss places additional systems requirements on the analog to digital converter and subsequent signal processing electronics.
Since the intrinsic behavior of these optical systems is known, it is possible to design into the focal plane array a corrective gain that is spatially correlated to the errors introduced from the optical system. This invention describes the methods and circuitry for performing this corrective gain processing for scanning and staring infrared focal plan arrays. Two specific circuit areas are described for this corrective processing. These are the areas of the transimpedance amplifier and the background signal charge skimmer.
There is prior art developed in the areas of the optical systems, electro-optics sensors, in the circuitry associated with these devices, but the specific circuitry developed and employed as presented in this application is deemed unobvious and novel. However, it should be noted that similar circuitry and methods within the scope of this invention could be used to realize the corrective signal processing for a wide range of electro-optical sensors and systems.
One object of the invention is to provide on-focal plane signal processing electronics for the correction of optically introduced non-uniformities applicable to infrared, visible and other electro-optical sensors in a wide range of applications and markets.
Another object of the invention is to allow the use of lower-cost, smaller and lower weight optical systems which are more likely to generate optical signal errors to focal plane.
A further object of the invention is to provide such optical systems which are benefit from the incorporation of such on-focal plane signal processing electronics.